Means for electro-erosion



June 18, 1957 L. R. BLAKE 2,796,509

MEANS FOR ELECTRO-EROSION Filed Dec. 9. 1954 s Sheets-Sheet 1 IN VEN 7-0;?

H T TOPNEW June 18, 1957 L. R. BLAKE 72,796,509

MEANS FOR ELECTRO-EROSION Filed Dec. 9, 1954 3 Shee'ts-Sheet 2 INVENTOR LESLIE RE6INI4LD BLHKE June 18, 1957 L. R. BLAKE 2,796,509

I MEANS FOR ELECTRO-EROSIQN Filed Dec. 9, 1954 3 Sheets-Sheet 3 47 TOP/1A6) United States Patent MEANS FOR ELECTRO-EROSION Leslie Reginald Blake, Rugby, England, assignor to The British Thomson-Houston Company Limited, a British company Application December 9, 1954, Serial No. 474,304

Claims priority, application Great Britain December 21, 1953 14 Claims. (Cl. 21969) This invention relates to apparatus for the working of material, particularly material of great hardness, by removing particles of it from the surface, utilising repeated electric discharges which act similar to the cutting effect of one or more sharp edges of a conventional tool. As compared with such conventional methods, machining by electro-erosion has many advantages. The tool electrode and its operating'head are of light weight, and results can be achieved which are not possible with ordinary methods, for instance holes of irregular cross sectional shape, or with curved axes can be drilled, or complex patterns can be engraved with a suitably shaped'tool electrode. Moreover, little or no mechanical or thermal stresses are set up by electro-erosion, and the work piece is left in a condition which is more strain-free than can be achieved with other methods.

The tool electrode can vibrate or rotate and may perform machining operations which are equivalent to drilling, milling, grinding, sawing and other conventional machining.

With conventional apparatus of this kind energy from a supply source is stored in a capacitor which is discharged in short pulses, the charging and discharging cycles being repeated at frequent intervals. It has also been known to connect a charging resistor between the condenser and the supply.

However the considerable loss of power in such a resistor can be avoided and the charging time can be reduced if an inductance device is included in the charging circuit, instead of or in addition to a resistor.

The present invention aims at an improved apparatus of this kind which is particularly suitable to be employed for high power output and machining rates. The invention also makes it possible to use simple control means, and affords a high charging efiiciency.

Generally this invention resides in apparatus for machaning by repeated electric discharges between tool and Work piece electrodes, comprising a capacitance device connected between the tool and work piece, an inductance device through which the capacitance device is charged from a source of electric power to supply the discharges, a vibrator for varying over a given range and in repeated succession the distance between the tool and work piece, the range including the distance at which the discharges occur at a predetermined voltage of the capacitance device as required, the charging circuit and the vibrator being synchronised to ensure coincidence of said voltage and distance, a means for feeding to the discharge gap a scavenging fluid for removing the particles eroded from the electrodes, and an automatic progressing means which is controlled independently of the voltage across the discharge gap to produce an automatic progressive motion reducing the distance between the tool and work piece, thereby to maintain substantially constant the mean length of the discharge path, while particles are eroded and removed from the electrode surfaces.

As will be seen such apparatus can be supplied from direct, current or alternating current sources, the removal compared with a reference voltage.

2,796,509 Patented June 18, 1957 ice of the eroded particles is greatly facilitated, and the self-feed mechanism can be of great ruggedness.

More details and advantages of the invention will become apparent from the following description referring to the accompanying drawings, in which:

Fig. 1 is a schematic representation of apparatus of conventional design using a resistance charging circuit for the capacitor and electronic control for the automatic progressing means;

Fig. 2 is a diagram of the capacitor voltage obtaining with such an arrangement;

Fig. 3 shows diagrammatically a form of apparatus according to the invention;

Fig. 4 shows another embodiment;

Fig. 5 shows the voltage graph of the capacitance device;

Figs. 6 and 7 show embodiments substantially similar to Fig. 3, and adapted for supply from an alternating current source;

Fig. 8 shows capacitance voltage and current curves obtaining in the resonant circuit when no discharge occurs, and

Fig. 9 shows the shape of these curves when discharges occur in the apparatus of Figs. 6 and 7; and

Fig. 10 shows a variation of the combined vibrator and induction device.

With conventional apparatus as shown diagrammatically in Fig. 1 repeated electric discharges of extremely short duration and high intensity which produce the electro-erosion effect between the electrode'or tool 1 and work piece 2 are supplied by a capacitor 3, arranged to be charged up via a resistor 4 from a direct current source. The work piece is connected to the positive side of the source and is usually earthed. The distance or gap between tool and work piece is so chosen that spark discharge occurs every time the charging capacitor 3 is charged up to a predetermined voltage. After the discharge of the capacitor the cycle of operation is repeated. A scavenging fluid which may be an electrolyte, but is preferably a dielectric material, such as a hydrocarbon, is fed to the discharge path so as to remove the eroded particles. The sparking distance between the electrodes approximately 0.001 to 0.005 inch for discharge voltages of 30 to 200 volts-is maintained by an electronic servo mechanism 5 supplied through an amplifier 6 whose input is dependent upon the voltage across the discharge gap as The latter can, be obtained from a potentiometer 8 having its ends 7 connected to a steady direct current source and its movable contact 9 connected to the work piece as shown. It will be understood that the sparking distance, discharge voltage and discharge repetition rate can be controlled by the setting of the reference voltage through the potentiometer.

While such an arrangement is satisfactory in many cases it has the disadvantage of necessitating an expensive and complicated, preferably an electronic equipment for controlling the progressive motion of the electrode 1. Furthermore the charging efliciency is poor since the capacitor voltage approaches asymtotically the value of the supply voltage and a major portion of the power drawn from the source is dissipated in the charging resistor. Optimum cutting power is achieved at 36% efiiciency when the discharge voltage V0 is 71% of the voltage Vs of the direct current source, but in practice the efiiciency can fall to less than 20%, which would mean that at a cutting output of 10 kilowatts the losses in the charging resistor may be as high as 40 kilowatts. A further disadvantage is that after the discharge a shortcircuit path may exist between the electrode and work piece for a period which is not negligible, causing delays in the charging operation, and non-uniformity in the peaks of the capacitor voltage at which discharge takes place. These voltage and current conditions are diagrammatically shown by the Vs and V graphs of Fig. 2.

The above disadvantages are overcome by the apparatus of the invention which ensures high .efficiency, ease.

of operation and constructional strength to withstand.

handling. in a workshop, such apparatus including aninductance device in the, charging circuit, a. means .for

mechanically vibrating.- the. electrode towards andaway from. the. work piece, anda means which is controlled independently of the voltage across thedischargegapior urgingthe electrode towardstheworkpiece andproducing an automatic progressive motion which. maintains substantially constant the, mean, length of the discharge gap while the eroded particles aremovedfrom thesurface.

charges, across the. gap between. theelectrode 1'. and Work. piece2 is charged up from a direct current source-through.

an inductance. device 10, comprising the energising coil of. an electromagnet.13..forming,part of.-a mechanical.

vibrator which is provided for varying througlpa. given. range and; in. repeated succession the gap, between 11 and 2. This range includesthedistance at which. discharges.

occur. The: charging circuit now. including inductance and capacitance has a natural resonance frequency- :1

The mechanicalv vibrationsand the natural frequencyof thevibrating system are related to-.this charging. frequency, the. frequency of the vibrations being preferably twice the. chargingfrequency. Furthermore, the-phase relation-is such that- .the. capacitance device is charged to a predetermined voltage when, dueto the vibrating'mo tion, the gap; which determines the-discharge path, passes through a conditionalvalue at which it will break down at the said predetermined voltage. The mechanical bias of-theelectrode-tool towards the workpiece and the pro-- gressingfeed ;of the electrode is obtainedby gravity-action usingasuitableportion of the weight. of a headcarrying-- the tooland vibratingmechanis'm, theremaining portion of. the. weight being-counterbalanced by a. weight" 12.:

Thus the-progressive motion does-not depend upon the' voltage across ltheelectrode gap and does not-necessitate-- electronic .control or servomotor means.

The operation-isnow as followss Theoretically, as-long as there is nodischarge-the voltage-of the capacitor in-'theresonant inductance-capacitance circuit oscillates about the: voltage Vs of thedirect current supply source; the current, also of sinusoidal shape, attaining-the value the surfaces of both electrodes as is known. If these surfaces are of'similar material the erosion of the positive electrode is greater but the actual amount of erosion depends upon the kind of material. The eroded material is rapidly removed by a scavenging fluid preferably conducted to the discharge path through a hollow of the electrode 1 and out of the discharge. end 1111,- aspreviously explained. It is fed from the supply tube 1a under pres.- sure which may be .as highas: 150 lbs.. per. squareinch. Various scavenging fluids have been tried with success and in our opinion a dielectric liquid, particularly a hydrocarbon, of the kind-used for insulating-and cooling transformersis very suitable. Thescavengingfluid does not only prevent shortcircuiting of the capacitor by accumulation of eroded particles in the gap between the electrode and work piece-but also causes-de-ionization of the gap thus quenching the discharge so that the recharging of the capacitance device can follow. It will be understood that the tie-ionization is-enhancedas; the

electrode'recedesfrom the work piece; inthecourseof its vibration, and that" this -receding movement is acceleratedbyarebounding action'consequential to the elec.- trode hitting the work piece; As astrong separating action isthus' ensured automaticrelative movement. of the -twoelectrodes-with a view to maintaining a more or less constant gap between them in spite of the removal of eroded material from their ends can.'be-readily accomplished by-a simple biasingmember controlling the 'automaticprogressing means; In the embodiment of-Fig. 3 the biasing is by'gravity force, using a smallportion of theweight of'thetool head includingthe vibrator, the greater portion'being counterbalanced by-a weight 12';

A manual control (not shown in the drawings), of'the tool head as is known in the artycan be provided-to override; the control by the automatic progressing. means.

It willbe understood that the capacitance value. of 3', the-frequency of the vibrator and the weight-12 which determines thebiasing force can be made adjustableby simple means. It, should also benotedthat although contact-betweenythe electrode and 'work piece must take place when the electrodehits the work piece the contact period is-extreme1yshort owing to the immediate reboundingetfect. Furthermore it does practi-callynot reduce the. charging efficiency since at the beginningof the charging period the actual charging current is kept at a low value by the. inductance device .in the charging circuit.

Whilst any reciprocating mechanism whose movement is suitably related to. the oscillationsin the charging cir- For bestoperations discharge occurs at the. first..voltage.

maximum of thecapacitance device, the value ofwhich. is now virtually twice the valueofjthe supply. voltage...

The simplest way. of. triggeringthedischarge at thisinstant is to .vibrate the electrode at .twice the.frequency of the chargingcircuit, and insuch phase relation thereto that the electrode approaches the workpiece. tobe at dis-. charge or sparking distancewhen the capacitance .device. attains or is near its peak voltage. As the electrode.

vibrates it moves onto the work piece but the capacitance,

device is substantiallydischarged before the electrode contacts'the workpiece, and'it rebounds immediately therefrom, its movement being similar to that of a -pneucuitcouldserveas. the mechanical vibrator, a combined inductancedevice-electromagnetic vibrator unit which .can readily be locked in phase with the charging circuit as shown in Fig. 3 is presentlypreferred, The tool 1 is carried by an armature 15. which is biased away from the magnet by a spring, preferably a leaf spring, which renders the constructioncompact and efficient.

The core 13 can have :two legseach carrying. aportion of. the winding 10, as in Fig. 4, or, asshown inFig. 10, a

three-legged core 13a may be usedwhich carries a single winding 10a on its centre leg.

The naturalfrequencyj" of.the vibrating system now depends uponits mass and. the. stiffness offthe spring. Obviously the attracting forcevaries; with the magnetic force whose frequency fma. is

A switching means such-as'a currentresponsive circuitbreakeror contactor 16 can be connected'to interrupt'at 17. the. F charging circuit if- -'the period of contact betweenthe tool and work piece is continued, and undesirable short circuit conditions arise. Immediately after interruption the contactor recloses and momentary heavy current in coil a pulls the tool away from the work piece. Thus a short circuit is cleared within a few milliseconds, thereby to prevent undesirable fusion which would be liable to cause the tool electrode to adhere to the work piece and would interfere with the smooth operation of the tool which requires constant vibration. Power stored during this period in the charging inductance device is released thereafter to the capacitance device. For normal operation of the embodiment shown in Fig. 3 the V value follows the curve of Fig. 5.

Obviously the direct current embodiment as shown in Fig. 3 can be supplied from an alternating current source via a rectifier, and, since the efficiency of apparatus according to the invention is very high, the size of such a rectifier, and of a transformer if used in conjunction therewith, is small as compared with the equipment required for conventional apparatus.

However, the invention also enables the operation of electro-erosion apparatus from an alternating current supply source without the intermediary of a rectifier. The charging circuit of such apparatus, as shown in Fig. 6, for instance, including capacitance device 3 and inductance devices 10 and 10 is oscillatory and, as indicated in Fig. 8, the voltage V and current I of the capacitance device build up in oscillatory fashion until discharge occurs. The charging circuit is now tuned to the frequency of the supply so that the charging frequency substantially equals the supply frequency, and the vibrator can also be synchronised to this frequency. As the vibrator comprises an electromagnet supplied from the same supply system polarization using a permanent magnet or a rectifier shunting a coil energised by A. C. is suggested to ensure that discharge occurs at the second voltage peak of the capacitance device, and at the instant when the work piece is positive.

If A. C. is used the natural frequency of the mechanical system may equal substantially that of the magnetising current, that is the charging frequency, and need not be twice the frequency of the charging circuit as with direct current charging.

The embodiment of Fig. 7 employs alternating current charging with a combined inductance device-vibrator unit, similar to the one used in the embodiment of Fig. 3. As can be seen from the voltage and current graphs of Fig. 9, the discharge now occurs at the second voltage maximum of the device 3, and this can be ensured by the provision of a polarizing winding 18 energized with direct current, so that the total magnetic flux in the core 13 oscillates at the frequency of the charging circuit. In lieu of such a direct current winding a permanent magnet core can be used to achieve the desired polarization characteristic.

In Fig. 6 the winding 10 is connected in series with another induction coil 10 in the charging circuit to provide the total of the inductance required in that circuit, and the winding It) is shunted by the rectifier, 19 to afford a polarized characteristic of the electromagnetic vibrator. In the modification shown in this figure, the scavenging fluid is discharged under pressure into the spark gap through separate nozzle means 24 which includes a device, such as a reducing valve 25, for regulating the pressure of the scavenging fluid. This regulable supply of scavenging fluid under pressure which is thus provided affords a means for obtaining a finer adjustment of the gap between the tool electrode 1 and the work piece 2 as the force of the scavenging fluid in the gap upon these members tends to force them apart and can be regulated by the supply of scavenging fluid.

As is known a more economical capacitor of higher voltage and less microfarad value can be used if a transformer stepping down the voltage is arranged between the capacitance device 3 and the electrodes 1, 2 and an automatic switching means, which may be of the mechanical or electronic kind, can be arranged to disconnect the transformer during the charging periods of the capacitance device.

With apparatus according to the invention it has been possible to use cutting powers of up to 10 or 12 kilowatts, the cutting power being defined by the formula V2CV f, wherein C is the capacitance of the device 3, V is the discharge voltage, and f is the discharge repetition frequency, to machine a work piece of tungsten carbide for instance at a rate of 0.5 cc. per minute, and to raise the efliciency of the charging operation to as compared with a usual value of the order of 20% for electro-erosion apparatus with resistance charging.

While some practical embodiments have been illustrated and described it will be understood that constructional modifications and different circuit arrangements can be used without departing from the invention. For instance, a spring device or a stalling electric motor can supply or regulate the biasing force of the automatic progressing means. The latter embodiment is illustrated in Fig. 4 in which a stalling motor 20 with an adjustable torque, which can be adjusted by means of a potentiometer 21 independently of the voltage across the gap between the tool electrode and the work, exerts a force which biases the electrode 1 towards the work piece 2 through a suitable drive, such as a pinion 22 in engagement with a rack 23. In this arrangement, the scavenging fluid is discharged into the gap through nozzles 24 which may be mounted in any suitable manner independently of the electrode 1. An electromagnetic vibrator could have a moving coil instead of the usual soft iron armature.

When a transformer is arranged between a capacitance device charged from a 'direct current source and the discharge gap, mechanical or electronic switching means, as are known, should be provided for disconnecting the transformer during the charging periods.

What I claim is.

1. Apparatus for electrically eroding a workpiece of metal or metallic compound by means of rapidly repeated electric discharges between it and a tool electrode, comprising a source of electric power, a capacitor connected to said tool electrode and workpiece to supply the energy of each discharge, a charging circuit for recharging the capacitor from said source after each discharge, said charging circuit having a minimum of ohmic resistance so as to be practically non-resistive and purely inductive, at least a part of its inductance being provided by a coil which serves for energizing in synchronism with the charging cycles of the capacitor an electromagnetic vibrator, said vibrator connected to said tool electrode to decrease and increase the distance between the said electrode and workpiece, thus cyclically varying the resistance in the discharge gap to initiate a discharge of considerable energy therethrough at the end of each recharge of the capacitor, also comprising a first means for feeding a scavenging fluid under pressure to the discharge gap, thereby to remove particles distintegrated by the electric discharges, and a second means producing a biasing force urging the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or by the tendency of the pressure fluid to force the tool electrode and workpiece apart, said biasing force being substantially constant during operation and independent of the fluctuating tool electrode voltage.

2. Apparatus including a tool electrode for electrically eroding a workpiece of metal or metallic compound by means of rapidly repeated electric discharges between it and said tool electrode comprising a source of electric power, an electromagnetic vibrator for said tool electrode, a capacitor connected to said tool electrode and workpiece to supply the energy of each discharge, a charging circuit for recharging the capacitor from said source after each discharge, said charging circuit having a minimum of ohmic resistance so as to be practically non- Iesistive and purely inductive, a cell for energising said.

7 r electromagnetic-vibrator in' Y synchronism'with the" charging cycles of" said capacitor and" providingzat least 'a" part of the inductance of said charging circuit,'said"vibrator being connected tosaid tool "electrode to decrease 'and increase the distance between the saidelectrodeandwork- *piece'and thereby cyclically varying'the resistance in the discharge gap 'therebetween 'to initiate 'a discharge of 'considerable energy :therethrou'gh" at the end'o'f ca'ch recharge of said capacitor, 'rneans'for feeding'a"scavenging fluid underpressure to the discharge gap for removing'nparticles disintegratedby the-electricdischarges, and a means for-producing and adjusting 'a' biasing *force urging said tool electrode towards' -theworkpiecetso'asto'prevent gap increase by the" erosion -of material or by the tendency ofthe pressure "fluid to force'said' tool electrode andworkpiece apart, said means-tor adjusting "the biasing force being adjustable independently of fiuctuations'of "the tool electrode voltage. V

f3. Apparatus*forelectricallyeroding a workpiece of metal or 'metallic' compound by "means of "rapidly repeated electric discharges between it-anda tool electrode, comprisingasource'of electric power, a'capacitor connected to said *toolc'lectrode and "workpiece to supply the energy'of each discharge,a charging circuit for recharging-the capacitor from said" source after each discharge, said charging circuit having a minimum-of ohmic resistanceso 'as 'to be practically non-resistive and 'purely in- 'ductive, at least'a part 'of "its inductance being provided bya 'coilwhich serves forenergising in synchronism with 'thebharging' cycles of the capacitor "an electromagnetic vibrator, said vibrator connected to 'said tool electrode to decrease and increase the-distance between the said electrode andworkpiece, thus cyclically varying the resistance in the dischargegap to initiate 'a discharge of considerable energy therethrough'at theend of each recharge of the capacitor,--also 'comprisinga means for feeding a scavenging fluid under pressure to the discharge gapg thereby to remove particles disintegrated by the electric dischargesjthe 'tool electrode being carried on a -tool' headand a weight being provided tobalance adjustably-a portion of the gravity force of said head, thus to produce a biasing force'urgingthe tool electrode towards the workpieces-o as to prevent gap increase by the-erosion of material or 'by the tendency of, the pressure'fluid to force the tool electrode and workpiece apart, said-biasing-forcebeing substantially constant during operation and independent of the fluctuating tool electrode voltage.

-4. Apparatus for electrically eroding a workpiece of metal or-metallic compound by means of'rapidly repeated electric discharges between it and a tool electrode, comprising an alternating current supply source, a capacitor connected to said tool electrode and'workpiece to supply :the energy of each discharge, a charging circuit for recharging the capacitor from said source after each dis- 'charge,'=said charging circuit having aminimum-of ohmic resistanceso asto be practically nonresistive and purely inductive, at least a part of its inductance being pro- 'vided by a-c'oil whose magneticfield is polarised by a unidirectional current conduc'tingdevice connected in'parallel thereto-which coil serves for energising in synchronism with'the charging cycles of the capacitor an electromagnetic vibrator,- said vibrator connectedto saidtool electrode to decreaseand increase the distance between the said elect'rode a'nd workpiece, thus cyclically varying the resist- 1 gasses 8, r substantially constant during operation aiiddn'deperident of thefluctu'ating-tool electrode voltage.

5. Apparatus for electrically eroding a workpiece of metal ormetallic compound by means of electric sparks produced between it-and'atool electrode, comprising'a source =of' elect ricpower, a capacitor connected to said tool' electrode --and--workpiece to supply the energy for 'the sparking, a charging circuit -"f0r recharging the capacit'or fromsaid sourceafter each discharge, said charging c'ircuit-h-avinga minimum of ohmic resistance so as to be practically non-resistive and purely inductive, at least a part of its-*inductance being provided by a coil which serves for energizing in synchronism with the chargingcycles 'ofthe capacitor an electromagnetic vibrator,=-said vibrator being connected to-said tool electrode -to decreaseiand increase the distance between the said electrode and -workpiece,-thus cyclically varying the resistance in'the discharge gap to initiate a spark discharge of considerable energy there/through at the end of each recharge of the capacitor, and alsocomprising'a first -means tori feeding a-scavenging fluid under pressure to the discharge gap, thereby to remove particles disintegrated by theelectric-sparking,and a second means producing- -andadjusting a biasing forceurging the tool electrode'-towards the workpiece-so as to prevent gap increase by' the erosionof material -01 by the tendency of the pressure fluid to force the tool electrode and workpiece-apart, the adjustmentof said-biasing force being independent =of fluctuations-of the tool electrode voltage.

Apparatus including a=toole1ectrode for electrically eroding a workpiece ofmetal or metallic compound by means of rapidly repeated electric spark discharges be- 'tween'the workpiece'and -said-tool electrode comprising a direct current-source,-an electromagnetic vibrator for said tool electrode,-a capacitor connected to said tool electrode *andworkpiece to supply the energy of each disc'harge, a'resonant charging circuit for recharging said capacitor from said source after each discharge, said charging circuit 'having a minimum of ohmic resistance so' as-to' be substantiallynon-resistive and purely-inductive, means including '-a coil for energizing said electromagnetic vibrator insynchronism with the charging cycles of said capacitor and providing at least'a part of the inductance of said charging circuit, said vibrator being connected-to said toolelectrode to decrease'and increase the ,distance 'between said electrode and workpiece at a frequency which approximates twice the frequency of said charging circuit and thereby cyclically varies the ef- *fective resistance in the spark discharge gap therebetween and initiates a jspark discharge of considerable energy 'therethrough at the end of each recharge of said capaci- :tor, 'means 'for feeding a scavenging fluid under pressure to the spark discharge gap for removing particles disintegrated by'the electric spark discharges, and means for producinga'biasing force'on said tool electrode in such manner and of such magnitude that the tool electrode is advanced towards the workpiece for preventing undesirable gap' increase by the erosion of material or by the tendency of the scavenging fluid to force said tool .electrode and workpiece apart, said biasing force producing means being operable independently of and not responsive-to 'fluctuationsofthe tool electrode voltage.

7. Apparatus for electrically eroding a workpiece of metal or metallic compound by -means of rapidly re- -peated electric discharge sparks between it and a tool electrode, comprising an alternating current supply source, a capacitor connected to said tool electrode and work- 'piece. tosupply the energy of the discharge sparks, a chargingicircuitfor recharging the capacitor from said source after each discharge, said charging circuit'having --a-minimum of ohmic resistance so as to be'practically non-resistive.andvpurely inductive, at least a part of its :inductancelbeingprovided by the magnet coil of an electromagnetic vibrator,'=operating-in synchronism with the charging cycles of the 'capaciton said. vibrator being connected to said tool electrode to decrease and increase at a frequency which equals approximately the frequency of the supply source, the distance between the said elec trode and workpiece, thus cyclically varying the resistance in the spark gap to initiate a discharge of considerable energy therethrough at the end of each recharge of the capacitor, and also comprising a first means for feeding a pressure fluid to the discharge gap, for removing therefrom particles disintegrated by the electric discharges, and a biasing means for urging the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or by the tendency of the pressure fluid to force the tool electrode and workpiece apart, said biasing means operating substantially independent of the fluctuations of the tool electrode voltage.

8. Apparatus for electrically eroding a workpiece of metal or metallic compound by means of rapidly repeated electric discharges between it and a tool electrode, comprising a source of electric power, a capacitor connected to said tool electrode and workpiece to supply the energy of each discharge, a charging circuit for recharging the capacitor from said source after each discharge, said charging circuit having a minimum of ohmic resistance so as to be practically non-resistive and purely inductive, at least a part of its inductance being provided by a coil which serves for energizing in synchronism with the charging cycles of the capacitor an electromagnetic vibrator, said vibrator connected to said tool electrode to decrease and increase the distance between the said electrode and workpiece, thus cyclically varying the resistance in the discharge gap to initiate a discharge of considerable energy therethrough at the end of each recharge of the capacitor, and also comprising a means for feeding a scavenging fluid under pressure to the discharge gap, thereby to remove particles disintegrated by the electric discharges, and an electric motor mechanically interconnected with the tool electrode and constructed to supply a stalling torque which provides a biasing force urging the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or by the tendency of the pressure fluid to force the tool electrode and workpiece apart, said biasing force being substantially constant during operation and independent of the fluctuating tool electrode voltage.

9. Apparatus for electrically eroding a workpiece of metal or metallic compound by means of a plurality of electric discharges produced in rapid succession between it and a tool electrode, said apparatus including an electric supply, a capacitor connected to said tool electrode and workpiece to provide the energy of each discharge, a charging circuit for recharging the capacitor from said source after each discharge, the ohmic resistance of said charging circuit being kept at a minimum so as to render said circuit practically non-resistive and purely inductive, at least a part of its inductance being provided by the energizing coil of an electromagnetic vibrator vibrating in synchronism with the charging cycles of the capacitor, said vibrator being mechanically connected to said tool electrode to decrease and increase the distance between the said electrode and workpiece, whereby cyclical variation in the dischage gap conductivity causes a succession of discharges of considerable energy at the end of each recharge of the capacitor, and also comprising a means for forcing into the discharge gap a scavenging fluid suitable to remove particles disintegrated by the electric discharges, also including a spring means arranged to urge the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or by the tendency .of said fluid to force the tool electrode and workpiece apart, said spring means functioning independently of the fluctuating tool electrode voltage.

10. Apparatus for electrically working metal or metallic compound using a rapid succession of electric spark discharges between it and a tool electrode, comprising an electric supply, a capacitor connected to said tool elec trode and workpiece to provide intensive discharges, a charging circuit for recharging the capacitor from said source after each discharge, said charging circuit being practically non-resistive but purely inductive, including a coil which serves to energize in synchronism with the charging cycles of the capacitor an electromagnetic vibrator, said vibrator reciprocating said tool electrode to decrease and increase the distance between the said electrode and workpiece, for cyclically varying the resistance in the discharge gap to initiate intensive discharges therethrough at the end of each recharge of the capacitor, and also comprising a regulable supply of scavenging fluid under pressure to the discharge gap, suitable for removing particles disintegrated by the electric discharges, and a biasing means for urging the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or by other tendency to force the tool electrode and workpiece apart, the operation of said biasing means being substantially independent of the fluctuating tool electrode voltage.

11. Apparatus for electrically working metal or metallic compound by means of a plurality of successive electric discharges between it and a tool electrode, comprising a source of electric power, a capacitor connected to the said tool electrode and workpiece to supply the energy of each discharge, a charging circuit for recharging the capacitor from said source after each discharge, said charging circuit being practically non-resistive and purely inductive, at least a part of its inductance being provided by a coil arranged to energize in synchronism with the charging cycles of the capacitor an electromagnetic vibrator, connected to said tool electrode to decrease and increase the distance between the said electrode and workpiece, thus cyclically varying the discharge gap resistance to initiate high intensity spark discharges therethrough at the end of each recharge of the capacitor, a switch being provided in said capacitor charging circuit and controlled to open automatically if the current exceeds a predetermined value which may cause undesirable fusion of material and obstruction to electrode withdrawal after the current is interrupted, a pressure fluid feed for scavenging the discharge gap and a biasing means for advancing the tool electrode towards the workpiece so as to prevent gap increase by the erosion of material or other undesired tendency to force the tool electrode and workpiece apart, said biasing means operating independent of the fluctuating tool electrode voltage.

12. Apparatus for working metal or metallic compound by electric sparking between it and a tool electrode, comprising a source of electric power, a capacitor connected to it via said tool electrode, and to the workpiece to supply the spark energy, a charging circuit for the capacitor being selected to be practically non-resistive but inductive, and including the energizing coil of an electromagnetic vibrator arranged to operate in synchronism with the charging cycles of the capacitor cyclically to vary the resistance in the discharge gap by changing the distance between the said electrode and workpiece to thereby initiate high intensity spark discharges at the end of each recharge of the capacitor, the tool electrode carried by a leaf spring in such fashion that it depends therefrom at a center point thereof while the ends of said spring are fastened to the electromagnet of the vibrator, and also means for directing a fluid under such pressure to the discharge gap that it scavenges therefrom undesired particles, and a biasing means urging the tool electrode towards the workpiece with a force as required to prevent gap increase by the erosion of material or other tendency, the operation of said means being independent of the fluctuating tool electrode voltage.

13. Apparatus as claimed in claim 7 wherein means including a permanent magnet core is provided to said vibrator for polarizing said vibrator to oscillate at the frequency of the charging circuit.

14. %ppai atus as el-aimed in claim 7, wherein ibewibrator' ha s -a soft-item -c0re polarized-by::an--e1ectric wind- References Cited in the file of :this patent UNITED STATES PATENTS 1,168,346 Thomson Ian. 18, 1916 1,497,071 Daykin June 10, 1924 10 2,079,310 Bennett May 4, 1937 2,089,213 Labadie Aug. 10, 1937 2,306,230 Somerville Dec.'.22, 1942 12 Welch. Nov. 28, :1944 *Whiteet a1. Mar. 20, 1945 Warwick .Sept. 25, "1945 Klemperer Oct. 19, 1948 Rud'orlf Oct. 17, 1950 Williams Feb.'10, 1953 'McKechnie Oct. 6,1953 Herzstark Mar. 23, 1954 FOREIGN PATENTS Great Britain May 24, 1950 

